An essential amino acid for an organism is an amino acid that cannot be synthesized by the organism from other available resources, and therefore must be supplied as part of its diet. Nine amino acids, including lysine and leucine, are generally regarded as essential for humans. Deficiencies of particular essential amino acids in certain major food crops have spurred efforts to improve the nutritional value of plants. One strategy to improve the nutritional value of plants relies upon traditional plant breeding methods. Another approach involves genetic manipulation of plant characteristics through the introduction of exogenous nucleic acids conferring a desirable trait.